This invention relates to a technology for evaluating an image forming apparatus, and in particular, to a method for evaluating gradation capable of exactly evaluating the gradation characteristics of an image forming apparatus and an image forming apparatus to which this method for evaluating gradation is applied.
It has heretofore been practiced that an image of a patient as a radiographic object is formed, for example, in a radiographic film composed of a silver halide photosensitive material, and is used in diagnosis by a medical doctor. However, a radiographic film is not convenient for storing, and because it is such one that an image is formed in it by chemical reaction, it has a problem that the deterioration of image is produced to a certain degree with the passage of time.
In contrast with the above, in recent years, it has been put into practice to devise a method of obtaining radiographic image information without using a radiographic film which is inconvenient for storing and handling. For example, it has been proposed a radiation image reading apparatus (Computed Radiography, hereinafter abbreviated as CR) in which an image signal is obtained by it that an imaging plate substantially composed of a stimulable phosphor substance is used, a radiation image is picked up as stimulated emission by using excitation light after it has been temporarily accumulated, and this emitted light is photoelectrically converted.
Further, also it has been proposed lately an apparatus for reading radiation image information by combining a radiation-phosphor or a radiation-photoconductor with a two-dimensional semiconductor detector such as a TFT switching device (Flat Panel Detector, hereinafter abbreviated as FPD).
Further, also it has spread an image-inputting apparatus other than a simple X-ray radiography such as an X-ray computed tomography (X-ray CT apparatus) or a magnetic resonance imaging apparatus (MRI apparatus). Further, it has become more and more frequent an opportunity when a plurality of medical image-inputting apparatus of different types to provide a medical image such as an ultrasonic image diagnosis apparatus, an electronic endoscope, and a retinal camera are used in a mixed manner in a hospital. These medical image-inputting apparatus (modality) provides image information as a digital signal in most cases.
On the other hand, when these medical images are diagnosed, it is mostly employed a method in which an image is observed as a hard copy recorded on a light-transmitting recording medium and/or light-reflecting recording medium. For a medical image-recording apparatus, which records medical image information on a recording medium, a method in which an image is recorded by exposing a recording medium using a silver halide recording material to a laser beam, a method of thermal color development, a method of thermal melt transfer, and a method of thermal sublimation transfer have been proposed. Further, lately, expectation has been placed on a possibility that a medical image is recorded by using a printer of an ink jet type having such an advantage as low cost. It has also become frequent an opportunity to use these medical image-recording apparatus of different recording methods in a mixed manner in a hospital.
Incidentally, there is a general evaluation that an image formed by an ink jet printer is easy to become granular to present a comparatively bad image quality. According to a study by the inventors of this invention, it has been found out that the granular roughness become remarkable to deteriorate image quality owing to the gradation of the image being not smooth. Further, also it has been found out that one reason of the gradation being not smooth is that an image is formed by using quasi-halftone processing in ink jet print method, and if this processing is not suitable, gradation does not become smooth in most cases. Accordingly, it can be said that the quality of an image formed by a printer can be improved by evaluating and improving the gradation characteristic of the printer.
As for the above-mentioned method for evaluating a gradation characteristic, there is such one that a jump in gradation and the degree of gradation loss are measured quantitatively by whether or not the difference between neighboring gradation levels in terms of lightness or density is under a certain threshold value. However, there is a problem that, in the case where a gradation curve has a shape of folded lines, a gap to be observed as existing at the fold portion cannot be evaluated by the above-mentioned evaluation method.